Implementing a Python-ish language is easy enough. Depending on your definition, Lua already is one. Implementing the totality of Python is very hard to do performantly. Anything you think you can cache can probably have that cache violated, and real code will tend to do it. For instance, surely you can just cache a class -> method table once? My goodness, no. The class can be dynamically modified, including rewriting what methods exist. This happens in, for instance, interpreter reloads, so it's a real problem. An instance can have a method dynamically overloaded. (Prototype inheritance is not supported by the language's style or keywords, but it is essentially supported by the semantic model.) Heck, an instance probably isn't what you think it is, check this out (tested in Python 2.7.4 and 3.3.1):

    >>> class A:
    ...     def method(self):
    ...             print("A")
    ...
    >>> class B:
    ...     def method(self):
    ...             print ("B")
    ...
    >>> a = A()
    >>> a.__class__
    <class '__main__.A'>
    >>> a.__class__ = B
    >>> a.method()
    B

"Aaaaaaaaa!" goes the interpreter implementor.

It's not the raw syntax that is hard, it's the mutability of everything, you know, a user gets an instance of something, adds an overload of the __add__ operator to just that instance which then proceeds to change the class of the other element of the addition operation... sure, it doesn't happen often but it happens often enough you can't just ignore it, unfortunately, or what you've implemented isn't Python anymore.